In a camera having a zoom function, a series of optical elements are disposed in a movable arrangement so as to vary the optical field of coverage. Ambient light gathered through an adjustable focal length lens is focused onto film to capture a photographic image. The 35 mm format film standard is the most commonly used image capture area. In such a camera, the adjustable focal length of the adjustable focal length lens can typically be varied in the range between 25 mm to over 100 mm. However, the amount of light passing through the lens decreases for a given lens aperture diameter as the lens focal length increases. This reduction in light passing through the lens is most pronounced in low-light photography. The reduced light transmission can result in underexposed photographs.
When the adjustable focal length lens is designed for the camera, the maximum diameter of the optical path defines the light gathering capacity of the lens. For example, in a camera with a maximum lens aperture of 7 mm, the amount of light transmitted at maximum aperture when the lens focal length is at 35 mm (a lens/aperture ratio commonly referred to as an f-stop of f/5) is twice the amount of light transmitted when the lens focal length is at 70 mm (a ratio commonly referred to as f/10). This change in light transmission might not be appreciated by all photographers, and photographs with incorrect exposure can be the result. If the f-stop becomes too large, image capture under low-light conditions becomes difficult without large auxiliary light sources. It is especially easy for the unwary photographer to select a long focal length without realizing the extent to which light transmission has been reduced. The problem can be overcome by increasing light path diameter, but this leads to unnecessarily large lens apertures at short focal lengths.
Cameras with flash units provide artificial scene illumination to help prevent underexposed photographs. Automatic exposure systems are known and used to adjust exposure and activate flash units, when necessary, without intervention by the camera user.
Even with automatic exposure systems and flash units, accidental underexposed photographs may still occur especially at longer lens focal length settings, where there is greatly reduced light transmission. Some cameras automatically select a shorter focal length, based on selected focal length and lighting conditions alone, other cameras operate to increase exposure time in low-light conditions.
A trend in the camera industry has been to minimize the size of the camera by minimizing the diameter of the adjustable focal length lens. However, the minimized diameter of the lens reduces its light gathering capability. On cameras having flash units and a relatively small maximum aperture, the effective flash distance is reduced as the lens is zoomed under low-light conditions. This problem increases as slower speed films are used.
Conventional films used to capture the photographic image vary in speed from ISO 64 to ISO 1000. The 35 mm format film standard incorporates a film canister that can be filled with film having any of these speeds. The camera responds to the film speed through operator input or by sensing information encoded on the 35 mm canister and the camera is typically designed to incorporate both slow and fast speed films. If the camera is provided with a flash unit, the flash unit must provide sufficient illumination for use with slower speed films. Typically, the flash unit is larger than needed for use with higher speed films.
Cameras which combine compactness, an adjustable focal length lens, or use only high speed film lose low-light picture taking capability when lower speed films are used. One known camera designed for ISO 400 film includes a 25 to 75 mm adjustable focal length lens and has a light gathering aperture of 10 mm. Maximum f-stop for this camera is 2.5 at 25 mm focal length and 7.5 at 75 mm. The camera includes a flash unit sized to a 6 foot range, allowing for acceptable exposure (2 stops under) out to 12 feet. The flash unit has a guide number (at ISO 400 ) of 15 at 25 mm focal length, and 45 at 75 mm focal length. If ISO 100 film is placed in the camera, the required guide number functionally doubles to 30 and 90 respectably at ISO 400. Since guide numbers are typically expressed at ISO 100, the required guide number for the flash system for ISO 100 film would range from 7.5 to 22.5 using ISO 400 film and 15 to 45 for ISO 100 film.
This situation presents a problem in sizing a flash unit for a camera that accepts various speed films. If the flash unit is sized to an intermediate guide number of 30 at ISO 100 , the unit is oversized for ISO 400 film and undersized for ISO 100 film at the longer focal length of 75 mm. A flash unit sized to accommodate all speeds of film at all focal lengths exceeds the limit of comfortable size for conventional camera designs.
Advanced cameras are electrically controlled and a series of control subprograms are executed to control camera operation. The control subprograms operate on information supplied from a light sensor and film speed sensor to control exposure of the film and have been used to vary the turn on light level for an electronic flash. In addition, the control subprogram has been used to control the shutter speed and aperture of the camera. A conventional exposure control subprogram operates on film speed, shutter speed and maximum aperture parameters to achieve best film exposure given the camera capacity. Cameras having better values for the parameters have a high price and large size. No camera systems consider film speed, shutter speed and aperture parameters as well as light levels and focal length to control exposure.